In integrated circuits (ICs), various components such as transistors, resistors, inductors, capacitors and varactors are configured to achieve the desired function. Varactors are essentially capacitors where the capacitance value varies with the voltage applied. One common type of varactors is metal oxide semiconductor (MOS) varactors. MOS varactors are generally employed for complementary MOS (CMOS) applications.
FIG. 1 shows a conventional n-type accumulation mode MOS varactor 100. The n-type MOS varactor is formed on an n-well 114 of a p-type silicon substrate 105. The varactor includes a gate structure 140 disposed on a surface 106 of the substrate. The gate structure includes a heavily n-doped (n+) polysilicon layer 142 over a dielectric layer such as an oxide layer 144. Contacts to the n-well are provided by n+ doped regions 148a-b. Typically spacers (not shown) are provided on the sides of the gate structure.
The capacitance of the varactor can be changed or tuned by varying an input voltage. The input voltage can be varied or swept between −Vdd to Vdd. This input voltage can be applied either to the polysilicon gate or to the n-well. When the voltage on the polysilicon gate is positive with respect to the n-well, negatively charged carriers accumulate in the substrate region under the gate. This causes the capacitance of the varactor to be at the maximum capacitance (Cmax). As the positive voltage on the polysilicon gate decreases and become negative with respect to n-well, negatively charged carriers in the substrate region under the gate migrate away, forming a depletion region, as indicated by dotted line 126. As width of the depletion region increases, the capacitance of the varactor progressively decreases. The depletion region reaches its maximum width when the gate is at the most negative voltage with respect to the n-well, which corresponds to the minimum capacitance (Cmin) of the varactor.
An important factor is the tuning range of the varactor, which corresponds to the capacitance range in which the varactor operates and is defined by ratio Cmax/Cmin. Generally, it is desirable for a varactor to have a large tuning range to provide better functionality of the circuit. Furthermore, particularly with mobile products, a high quality factor is desirable since this correlates to low power consumption.
From the foregoing discussion, it is desirable to provide a MOS varactor with a larger tuning range and a higher quality factor.